The present invention is directed to bicycles and, more particularly, to a shift control apparatus that selectively restricts one or more speed stages of a bicycle transmission.
Some bicycles are equipped with shift control devices that automatically control the operation of front and rear transmissions (e.g., front and rear derailleurs that respectively cooperate with a plurality of front and rear sprockets) depending on bicycle velocity. An example of such a system is disclosed in JP 8-501,742. In that system, the bicycle chain may be engaged automatically with any combination of front and rear sprockets depending upon the velocity of the bicycle. The use of all sprocket combinations better accommodates more sensitive changes in riding conditions than is possible by operating the rear transmission alone and also avoids unnecessary shifting.
While the ability to use all sprocket combinations allows the bicycle transmission to respond to more sensitive changes in riding condition, such fine response also may result in frequent shifting of the front and/or rear transmissions. Since in bicycles the operation of the front transmission results in a greater change of gear ratio than the operation of the rear transmission, operation of the front transmission exerts a greater shock on the rider's legs than operation of the rear transmission. Thus, frequent shifting by the front transmission to maintain the bicycle at a constant velocity runs the risk of imposing a considerable burden on the rider. On the other hand, frequent shifting by the front transmission to accommodate changing conditions when traveling uphill or on a rough road usually does not imposes a significant burden on the rider, and may even be beneficial. Thus, it may desirable to have the ability to control the operation of the front transmission to accommodate such different riding conditions. Of course, it also may be desirable to similarly control the operation of the rear transmission or both the front and rear transmissions to accommodate different riding conditions.